Many oil field applications now gather or would benefit from the gathering of information from the bottom of a well and the transmission of such information to the surface As well exploration, drilling, and other activities become more sophisticated, the demands for such down-hole information increase. The continually increasing sophistication and capabilities of electronics makes it possible to gather information of great precision and importance. Unfortunately, the transmission of data from down-hole to the surface has continuously been a major obstacle in obtaining such information.
In many cases a drill string is removed from a well, a logging tool is lowered into the well by a logging cable, and information is transferred between the logging tool and the surface by electrical conductors in the logging cable. While this surface-to-logging tool electrical cable permits the collection of high quality information from the well, it suffers some serious drawbacks. For example, this procedure is often undertaken only at extreme expense, loss of time, and loss of production. If it is employed in an open-hole situation there is always the danger of damage to the hole or loss of the tool. Moreover, it simply cannot be used to capture real-time data while drilling, which the industry describes as measurement while drilling (MWD). Furthermore, it cannot be used during well stimulation or fracturing where the logging cable would be damaged from fluids and fluid-conveyed particulate matter, or where it would represent an intolerable restriction of the fluid path.
Accordingly, alternate data transmission techniques have been developed to transmit information. Such procedures include modulating mud pressure, attaching or embedding insulated electrical conductors in a drill string, transmitting modulated acoustic waves, and transmitting modulated electromagnetic waves. While all of these techniques have been successfully employed in particular situations, none have proven practical for a wide variety of applications.
Well stimulation and fracturing operations are typically performed in cased wells where metal casing has been cemented in place to isolate production fluids from surrounding formation fluids. In these operations, mud pressure telemetry often fails to work because it is performed through tubing, which often requires use of packers, and is subject to interference caused by pressure pulsations from surface pumps. Likewise, the attaching or embedding of insulated electrical conductors in a drill string fails to work reliably because of poor electrical connections between drill string sections and cable wear from fluids and particles transported by the fluids. The transmission of acoustic waves suffers from a poor signal-to-noise ratio. Thus, the transmission of acoustic waves fails to work during acoustically noisy operations.
Moreover, conventional techniques for transmitting electromagnetic (EM) waves generally have not been successful in case-d wells due to the extremely low resistance and high magnetic permeability of the steels used in oil field casing. Conventional down-hole EM transmitters often utilize a large transformer for coupling a modulated electrical signal to the earth's bulk (lithosphere), for impedance matching, and for current amplification. While such a conventional EM transmitter may be configured with a minimally dimensioned transformer and centralizer-type electrodes to couple a high current, low voltage output signal into a well casing, the resulting output signal exhibits a voltage too low to be useful in all but the most shallow wells. This limited signal voltage results from the low casing resistance, undesirably high transformer impedance, and power amplifier inefficiencies. Moreover, such a conventional EM transmitter is powered from batteries, which demonstrate an undesirably short life when high-power output signals are transmitted though the inefficient power amplifier. Furthermore, such a conventional EM transmitter exhibits an undesirably low reliability because of excessive heat build-up in the high power devices employed by the inefficient power amplifier.
In order for the transmission of EM waves to effectively convey information from a logging tool positioned in a well to the surface, increased current must be injected into the casing so that a larger signal voltage will result. However, increased current from the coupling transformer requires a larger transformer design, and larger transformers typically cannot be accommodated in a well logging tool. On the other hand, prior art devices have been used to directly couple electrical currents into a well casing without using a transformer. However, such devices tend to utilize linear power amplifiers to inject small currents. Although often highly accurate, linear power amplifiers are particularly inefficient. Consequently, such amplifiers would suffer seriously degraded reliability and battery life if adapted to a high-power EM transmission application.